Fix overflow_check CompileError with msvc

This fixes 4 test errors with Cython 0.18 rc1 when using msvc compilers. Verified on win-amd64-py2.7 with msvc9 compiler.

The compiler error is:
```
overflow_check_int.cpp(4242) : error C2668: 'sqrt' : ambiguous call to overloaded function
        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\math.h(581): could be 'long double sqrt(long double)'
        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\math.h(533): or       'float sqrt(float)'
        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\INCLUDE\math.h(128): or       'double sqrt(double)'
        while trying to match the argument list '(__pyx_t_18overflow_check_int_INT)'
```

The test errors are:
```
======================================================================
ERROR: runTest (__main__.CythonRunTestCase)
compiling (cpp) and running overflow_check_int
----------------------------------------------------------------------
Traceback (most recent call last):
  File "runtests.py", line 727, in run
    self.runCompileTest()
  File "runtests.py", line 531, in runCompileTest
    self.test_directory, self.expect_errors, self.annotate)
  File "runtests.py", line 710, in compile
    self.run_distutils(test_directory, module, workdir, incdir)
  File "runtests.py", line 670, in run_distutils
    build_extension.run()
  File "X:\Python27-x64\lib\distutils\command\build_ext.py", line 339, in run
    self.build_extensions()
  File "X:\Python27-x64\lib\distutils\command\build_ext.py", line 448, in build_extensions
    self.build_extension(ext)
  File "runtests.py", line 285, in build_extension
    _build_ext.build_extension(self, ext)
  File "X:\Python27-x64\lib\distutils\command\build_ext.py", line 498, in build_extension
    depends=ext.depends)
  File "X:\Python27-x64\lib\distutils\msvc9compiler.py", line 552, in compile
    raise CompileError(msg)
CompileError: command '"C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\BIN\amd64\cl.exe"' failed with exit status 2

======================================================================
ERROR: runTest (__main__.CythonRunTestCase)
compiling (cpp) and running overflow_check_longlong
----------------------------------------------------------------------
Traceback (most recent call last):
  File "runtests.py", line 727, in run
    self.runCompileTest()
  File "runtests.py", line 531, in runCompileTest
    self.test_directory, self.expect_errors, self.annotate)
  File "runtests.py", line 710, in compile
    self.run_distutils(test_directory, module, workdir, incdir)
  File "runtests.py", line 670, in run_distutils
    build_extension.run()
  File "X:\Python27-x64\lib\distutils\command\build_ext.py", line 339, in run
    self.build_extensions()
  File "X:\Python27-x64\lib\distutils\command\build_ext.py", line 448, in build_extensions
    self.build_extension(ext)
  File "runtests.py", line 285, in build_extension
    _build_ext.build_extension(self, ext)
  File "X:\Python27-x64\lib\distutils\command\build_ext.py", line 498, in build_extension
    depends=ext.depends)
  File "X:\Python27-x64\lib\distutils\msvc9compiler.py", line 552, in compile
    raise CompileError(msg)
CompileError: command '"C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\BIN\amd64\cl.exe"' failed with exit status 2

======================================================================
ERROR: runTest (__main__.CythonRunTestCase)
compiling (cpp) and running overflow_check_uint
----------------------------------------------------------------------
Traceback (most recent call last):
  File "runtests.py", line 727, in run
    self.runCompileTest()
  File "runtests.py", line 531, in runCompileTest
    self.test_directory, self.expect_errors, self.annotate)
  File "runtests.py", line 710, in compile
    self.run_distutils(test_directory, module, workdir, incdir)
  File "runtests.py", line 670, in run_distutils
    build_extension.run()
  File "X:\Python27-x64\lib\distutils\command\build_ext.py", line 339, in run
    self.build_extensions()
  File "X:\Python27-x64\lib\distutils\command\build_ext.py", line 448, in build_extensions
    self.build_extension(ext)
  File "runtests.py", line 285, in build_extension
    _build_ext.build_extension(self, ext)
  File "X:\Python27-x64\lib\distutils\command\build_ext.py", line 498, in build_extension
    depends=ext.depends)
  File "X:\Python27-x64\lib\distutils\msvc9compiler.py", line 552, in compile
    raise CompileError(msg)
CompileError: command '"C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\BIN\amd64\cl.exe"' failed with exit status 2

======================================================================
ERROR: runTest (__main__.CythonRunTestCase)
compiling (cpp) and running overflow_check_ulonglong
---------------------------------------------------ALL DONE
-------------------
Traceback (most recent call last):
  File "runtests.py", line 727, in run
    self.runCompileTest()
  File "runtests.py", line 531, in runCompileTest
    self.test_directory, self.expect_errors, self.annotate)
  File "runtests.py", line 710, in compile
    self.run_distutils(test_directory, module, workdir, incdir)
  File "runtests.py", line 670, in run_distutils
    build_extension.run()
  File "X:\Python27-x64\lib\distutils\command\build_ext.py", line 339, in run
    self.build_extensions()
  File "X:\Python27-x64\lib\distutils\command\build_ext.py", line 448, in build_extensions
    self.build_extension(ext)
  File "runtests.py", line 285, in build_extension
    _build_ext.build_extension(self, ext)
  File "X:\Python27-x64\lib\distutils\command\build_ext.py", line 498, in build_extension
    depends=ext.depends)
  File "X:\Python27-x64\lib\distutils\msvc9compiler.py", line 552, in compile
    raise CompileError(msg)
CompileError: command '"C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\BIN\amd64\cl.exe"' failed with exit status 2
```